Acoustic signals are used for many different things. In marine environments, acoustic signals are often used to map a region, such as the sea floor, or to navigate, discover, indentify, and/or determine the range of an object such as a ship or submarine. In such marine applications where early discovery and/or identification of a ship or submarine is often desired, a sonar array is usually positioned in the water to monitor the region.
To discover and identify a vessel, the sonar array often includes a component that generates an underwater acoustic signal, and another component that senses the signal after the signal is reflected by an object in the region. For example, the sonar array 10 shown in FIG. 1A includes an explosive charge 12 (here two) to generate the underwater acoustic signal, and a buoy 14 that holds a component (not shown in FIG. 1A) for sensing underwater acoustic signals. To look for a vessel located within the region of the sonar array 10, one of the explosive charges 12 is released from its tether and exploded to generate a signal. The signal propagates through the water and reflects of off objects (not shown in FIG. 1A) in the region. The sensing component then senses the acoustic signal that is reflected back to buoy 14.
Unfortunately, using explosives to generate an underwater acoustic signal for the purpose of early discovery of a ship or submarine may be dangerous and/or ineffective. And the power of the signal may be often hazardous to the wildlife in the region. In addition, because the signal does not persist for a significant period, the signal only provides information about objects in the region at a specific moment in time; the signal does not provide information over a significant period of time. Thus, for example, a submarine traveling through the region is more likely to travel out of detection range between the times that a signal is generated, and consequently avert detection.
To overcome these shortcomings, some sonar arrays include an electro-acoustic transducer, such as a ceramic transducer, to generate acoustic signals under water for a longer period of time. For example, the sonar array 16 shown in FIG. 1B includes such an electro-acoustic transducer 18, and a buoy 20 that holds a component (not shown in FIG. 1B) for sensing underwater acoustic signals. To generate acoustic signals, the transducer 18 is typically powered by a battery (not shown in FIG. 1B).
Unfortunately, using a battery powered transducer to generate an underwater acoustic signal for the purpose of early discovery of a ship or submarine may be ineffective too. The life of the battery may be too short for some applications. Also, many batteries cannot provide enough power to generate a high-amplitude acoustic signal for a significant period. A high-amplitude acoustic signal might be desirable for many different reasons. For example, in a noisy littoral environment, one may need to generate a high-amplitude signal so that the signal can be distinguished in the noisy environment. Thus, in a noisy environment, such as in a littoral region, there may exist significant periods during which the signals generated by the transducer 18 are not effective. In addition, the electro-acoustic transducer may be unable to provide adequate pressure levels due to the physical constraints of the material (e.g., ceramic) from which the transducer is formed and/or the physical constraints in the environment such as the water pressure at substantial depths.